1. Field of the Invention
The present invention is directed to a method for examining a living subject with a non-invasive imaging method including obtaining the time curve of the concentration of a contrast agent having a physical property identifiable with the imaging method in a region of interest (ROI) of the subject presented in time-successive images acquired with the imaging method by identifying quantitative values of the physical property.
2. Description of the Prior Art
Methods of the above type are employed, for example, in x-ray computed tomography in order to be able to start the actual examination given the presence of a specific contrast agent concentration in the ROI.
For this purpose the operator marks, for example, a circular ROI containing the critical anatomical region in a reference image registered without the contrast agent being present, and the time-successive images are registered with the beginning of the administration of contrast agent. The average value of the physical property is determined for each image with respect to the ROI and is compared to the corresponding average of the image registered without the contrast agent. When the average exceeds a threshold that the operator can set, then the actual examination is triggered.
This procedure involves a number of disadvantages.
A first of these is that when the critical anatomical region is small, it is difficult to mark the ROI, moreover the critical anatomical region often cannot be clearly recognized until a certain contrast agent concentration has been reached. Another is that it is difficult under certain circumstances to discern the critical anatomical region only on the basis of anatomical information, for example, tumor-free and vessel-free zones of the liver. Additionally, when the critical anatomical region moves, for example as a consequence of swallowing motions or as a consequence of body movement on the part of the patient, the average of the physical property can be subject to substantial fluctuations, particularly when anatomical structures are located in the proximity of the ROI that exhibit substantial deviations in the value of the physical property from the average of the physical property exhibited the ROI.
An object of the invention is to provide a method of the type initially described that simplifies the marking of the ROI and is less sensitive to motion.
Thus, in the inventive method, the contrast agent concentration, preferably the time curve of the contrast agent concentration, is not determined on the basis of averages but on the basis of histograms. Differing from the case o observing averages, the increase in the contrast agent concentration in the ROI can be clearly recognized because not only an average but also the frequency of occurrence of the quantitative value of the physical property that corresponds to the contrast agent is identified for the ROI, by means of the histograms. This is even true when the ROI is significantly larger then the critical anatomical region situated within the ROI and wherein the contrast agent accumulates.
Thus, for example, there is the possibility of determining the time curve of the position of a straight line that proceeds parallel to the frequency of occurrence axis of the histogram, and that separates a region of the histogram containing a defined portion of the overall area of the histogram. This determination serves as a criterion for the time curve of the contrast agent concentration. Alternatively, there is also the possibility of determining the time curve of the position of a straight line proceeding parallel to the frequency of occurrence axis of the histogram and through the average of a region of the histogram containing a defined portion of the overall area of the histogram separated by this straight line. This determination also serves as a criterion for the time curve of the contrast agent concentration. In both instances, it is possible to determine the time curve of the contrast agent concentration itself even when the critical anatomical region is practically unrecognizable without the presence of a specific contrast agent concentration, by merely being able to mark a ROI that contains the critical anatomical region. Which of the two said possibilities is preferable in a specific application is dependent on the ratio of the size of the marked ROI to the size of the critical anatomical region, and on the ratio of the expected contrast agent concentration to the noise amplitude.
The defined portion of the overall area of the histogram that is separated corresponds to values of the physical property that lie above the value of the physical property that is dominant given the absence of contrast agent when the value of the physical property belonging to the contrast agent, as is normally the case, lies above the value of the physical property that is dominant given the absence of contrast agent. The defined portion corresponds to values of physical property that lie below the value of the physical property that dominates given the absence of contrast agent when the value of the physical property belonging to the contrast agent lies below the value of the physical property that dominates given absence of contrast agent.
If it is known in advance that no anatomical regions in which a high contrast agent concentration can be anticipated are within the ROI, in a version of the invention that the time curve of the average or median value of a central region of the histogram is determined as a criterion for the contrast agent concentration. This central region is limited by a straight line proceeding parallel to the frequency of occurrence axis of the histogram and separating a region of the histogram containing an upper, defined part of the overall area of the histogram, and by a straight line proceeding parallel to the frequency of occurrence axis of the histogram and separating a region of the histogram containing a lower defined portion of the overall area of the histogram. The upper region containing the defined portion of the overall area of the histogram represents values of the physical property that lie above the values of the physical property corresponding to the middle region of the histogram. The lower region containing the defined portion of the overall area of the histogram represents values of the physical property that lie below the values of the physical property corresponding to the middle region of the histogram.
In a further embodiment of the invention, the determination of the time curve of the contrast concentration can include producing a histogram with respect to an image obtained before the administration of contrast agent, this histogram being compared, for example by subtraction, to the histograms belonging to the time-successive images. The time curve of the spacing between a value of the physical property belonging to the contrast agent and the value of the physical property that dominates in absence of the contrast agent is determined as criterion for the time curve of the contrast agent concentration. The histogram relating to the image produced before the administration of contrast agent can be subtracted from the histograms belonging to the successive images. In this case, the value of the physical property belonging to the contrast agent generates a positive local maximum and the value of the physical property that dominates in the absence of contrast agent generates a negative local maximum in the resultant histogram that is derived by the subtraction. The interpretability of the resultant histogram can be improved when the histogram resulting from the subtraction is convolved with the histogram with respect to the image produced before the administration of contrast agent.
According to a modification of the invention, the time curve of the contrast agent concentration is utilized to start an examination when the central contrast agent concentration exceeds a threshold.
The inventive method is particularly suited for x-ray computed tomography, wherein a substance having very high absorption is usually introduced into the blood circulation as the contrast agent. Substances having a high atomic number, for example iodine (I 53) are frequently employed as the contrast agent.
In order to keep the x-ray dose administered to the patient before the actual examination as low as possible in the case of x-ray computed tomography, a preferred embodiment of the invention the time successive images acquired dependent on quantitative of the physical property are generated with a reduced x-ray dose compared to the x-ray dose employed in the implementation of the actual examination.
The invention is explained below with reference to the example of an exemplary embodiment relating to x-ray computed tomography.